I am having issues with many of my AVI files not playing "Cannot Play" what format should I use and can someone recommend a freeware converter or just one I can try to find to use?

What operating system, as this makes a small difference.

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I am interested in this too. Mainly the format. I am using windows 7 x64.

Use "Format Factory" and convert your avi's to mp4 with aac audio and they'll play
It's free.... http://www.formatoz.com/download.html

Aiseesoft Total Video Converter help you to convert all your videos from internet, camera and so on to the right video format you want.

1. Convert MP4, H.264, AVI, MP3, WMV, WMA, FLV, MKV, MPEG-1, MPEG-2, 3GP, 3GPP, VOB, DivX, Mov, RM, RMVB, M4A, AAC, WAV

2. Work perfectly with PSP, iPod Classic, iPod Touch, iPod Nano, iPhone (3G), iPhone, Zune, Zune 2, Blackberry, Nokia, Creative Zen, Sony Walkman, iRiver PMP, Archos, PS3, Apple TV, Xbox, iPAQ, Pocket PC, Mobile Phone

I've ever have many AVI files also, but they can not be played on my iPhone, so I use this videos converter to tranform them. And it works. Unfortunately, this tool is a paid one.
For free tools, you can try freemake.